Method of electroplating gold and gold plating baths containing an amido-polyphosphate

ABSTRACT

Disclosed is an aqueous composition suitable for plating gold deposits of improved brightness, and a method of plating therewith. An amido-polyphosphate compound is added to a plating bath containing a gold complex, and the resulting deposits exhibit a substantial improvement in brightness. The visual quality of the deposits improves further by the additional inclusion of a polyphosphate compound. Still further improvements may be obtained by the inclusion in a bath containing a gold cyanide complex of a small quantity of cobalt or nickel ion. In addition to improved brightness, the latter bath also exhibits improved hardness, an important characteristic for many applications.

United States Patent 1191 Deuber et al.

[ Aug. 5, 1975 [75] Inventors: John M. Deuber, Nutley, N.J.;

George R. Lurie, Huntington Beach, Calif.

[73] Assignee: Oxy Metal Industries Corporation,

Warren, Mich.

[22] Filed: Apr. 19, 1974 [21] Appl. No.: 462,268

[52] US. Cl 204/43 G; 204/46 G [51] Int. Cl. C23b 5/28; C23b 5/42 [58] Field of Search 204/43 G, 46 G, 44

[56] References Cited UNITED STATES PATENTS 2,122,122 6/1938 Woodstock 210/23 FOREIGN PATENTS OR APPLICATIONS 928,088 6/1963 United Kingdom 204/46 G 6,616,173 5/1967 Netherlands 204/46 G Primary ExaminerG. L. Kaplan Attorney, Agent, or FirmArthur E. Kluegel; Richard P. Mueller; B. F. Claeboe [5 7 ABSTRACT Disclosed is an aqueous composition suitable for plating gold deposits of improved brightness, and a method of plating therewith. An amido-polyphosphate compound is added to a plating bath containing a gold complex, and the resulting deposits exhibit a substantial improvement in brightness. The visual quality of the deposits improves further by the additional inclusion of a polyphosphate compound. Still further improvements may be obtained by the inclusion in a bath containing a gold cyanide complex of a small quantity of cobalt or nickel ion. In addition to improved brightness, the latter bath also exhibits improved hardness, an important characteristic for many applications.

16 Claims, No Drawings METHOD OF ELECTROPLATING GOLD AND GOLD PLATING BATHS CONTAINING AN AMlDO-POLYPIIOSPHATE BACKGROUND OF THE INVENTION This invention relates to the art of plating gold deposits. More specifically, it relates to the electrodeposition of gold deposits exhibiting improved brightness.

Numerous plating baths have been proposed for the electrodeposition of gold deposits. Foremost are those baths containing the gold as a cyanide complex, and those containing gold as the sulfite complex. Pertinent to the cyanide baths are U.S. Pat. Nos. 2,905,601; 3,373,094; 2,765,269; 2,812,299; and Re. 25,883. The sulfite complex baths are described for example in U.S. Pat. Nos. 3,057,789 and 3,666,640.

The foregoing patents describe generally the parameters for plating using the gold complex compounds, and describe various brightening agents which may be included in the plating baths. Suggested brighteners include hydrazine and piperazine derivatives, as well as small quantities of arsenic, antimony, and selenium ions.

As generally practiced, the gold cyanide baths are adjusted to a pH of 3.5 or greater. The sulfite baths are adjusted to a pH of from 5.5 to 10.5 and often rely on the presence of an amine compound in order to maintain stability at the lower pl-ls.

SUMMARY OF THE INVENTION According to the present invention, an aqueous gold plating bath containing gold in the form of a complex is provided with a quantity of an amido-polyphosphate compound sufficient to improve the brightness of the deposits obtained by electroplating therewith. In a preferred embodiment, the aqueous plating solution additionally contains at least 1 g/l of a polyphosphate com pound. In the case of a cyanide complex bath, a further improvement in brightness and an improvement of hardness in the deposit can be obtained by further including at least 0.01 g/l of cobalt or nickel ion.

DETAILED DESCRIPTION OF THE INVENTION In accordance with the state of the art, gold plating solutions commonly contain between 1 and 100 g/l gold in the form of a complex. In order to minimize losses due to dragout, the concentration of gold is preferably maintained between 1 and g/l. In the case of the sulfite complex, the gold is normally provided as an ammonium, amine, or alkali metal gold sulfite. In the case of the cyanide complex, the gold is normally provided in the form of an ammonium or alkali metal gold cyanide. For solubility reasons, the potassium salts are the most preferred. Common alloying metals may also be present in the solution.

The essential component of the plating baths of the present invention is an amido-polyphosphatc compound. This compound differs from the normal polyphosphate compounds in that one or more 'of the oxygen linkages between phosphorus atoms has been replaced by an NH- linkage. As is the case with polyphosphate compounds, the amido-polyphosphates may be added either as the ammonium or alkali salts. Compounds of this type may be made in various ways, for example, by the reaction of dry ammonia with a phosphoric anhydride compound as described in U.S. Pat. No. 2,122,122. These compounds may be made by reacting anhydrous ammonia with phosphorus pentoxide and heating the reaction mixture in the presence of excess anhydrous ammonia at a temperature of at least about 150C. for a sufficient period of time to effect a molecular condensation with loss of ammonia from the product, forming a substantially neutral product in which the atomic ratio of nitrogen to phosphorus is less than 1.5 to 1. The amido-polyphosphates have a P 0 equivalent phosphorus content of from 65 to percent by weight and an NI-l equivalent amide nitrogen content of from about 5 to 12 percent by weight. Where the ammonium salt is employed, the NI-l equivalent ammonium nitrogen content should be from about 8 to 22 percent by weight. One commercially available ammonium salt of an amide-polyphosphate is sold by the Stauffer Chemical Company under the trademark Victamide. The product contains approximately 76 percent P 0 equivalent phosphorus, 7 percent Nl-I equivalent amide nitrogen, and 15.4 percent NI-I equivalent ammonium. The compound exhibits an average phosphorus content of about 4 to 5 atoms per molecule.

The precise concentration of the amidopolyphosphate compound in the gold plating solution depends to some extent on the nature and concentrations of the other components present. It appears, however, that at least 1 g/l is necessary to obtain any appreciable effect and a quantity in excess of g/l does not provide any further improvement in brightness. In general, sufficient brightening effect can be obtained by employing quantities between 5 and 40 g/l.

In a preferred embodiment of this invention, it has been found desirable to include a polyphosphate compound in the solution as well. The amidopolyphosphate and the polyphosphate apparently coact in some matter to further enhance the brightness of the deposit. When it is desired to include the polyphosphate compound in the solution, at least 1 g/l should be included in order to obtain beneficial effects. Examples of suitable compounds include the ammonium and alkali metal salts of the pyrophosphate, tripolyphosphate, and tetrarnetapolyphosphate anions. Quantities in excess of 20 g/l are preferred.

When the plating solution contains the gold cyanide complex in the acid pH range, a still further preferred embodiment is the inclusion of a small quantity of cobalt or nickel in depositable form. While addition of the amido-polyphosphate and the polyphosphate compound improve the brightness of the deposit, the final inclusion of cobalt and nickel ion brings the deposit to full brightness. The inclusion of cobalt or nickel is thus preferred wherever brightness is of extreme importance as in the case of jewelry manufacture. As demonstrated in the examples, the inclusion of cobalt and nickel has the additional advantage of improving the hardness of the gold deposit.

The precise concentration of cobalt and nickel ions required in order to obtain the foreoging advantages depends upon the concentration of the other components of the solution. Since it is desirable to minimize the amount of cobalt or nickel in the final deposit, it is, of course, desirable to include no more cobalt in the solution than is necessary to obtain the desired brightness. The minimum cobalt concentration required appears to vary in proportion to the concentration of the amidopolyphosphate compound in the solution. Cobalt concentrations of 100 mg/l and less have been found effective, whereas amounts as high as l.] g/l have also been found effective without increasing the cobalt content of the deposit beyond 0.5 percent.

In order to simplify reporting in the following examples, a numerical scale has been adopted in order to describe the degree of brightness exhibited by a gold deposit:

1. Bright Details of a reflected image are distinct.

2. Semi-bright A reflected image is visible, though hazy.

3. Lustrous The deposit exhibits a low-level reflectance, although details of an image are not visible.

4. Dull, smooth yellow The deposit appears smooth, although there is no ordered light reflec- Plating was performed on cathodic brass panels at approximately 120F. and 3 ASP with periodic replenishment for a period of over two days. Deposits obtained exhibited an average visual rating of 1. The use of the pyrophosphate improves the visual quality as compared to the system containing neither the amidopolyphosphatc nor the pyrophosphate (4 rating) and as compared to the system containing only the amidopolyphosphate (2 rating) in Example I. ldentieal results were obtained where Na P O and Na P O were substituted for the sodium pyrophosphate. An additional advantage is that the pyrophosphate permits one to maintain the brightening effect for a longer period of time and buffers the solution to impart stability.

tion. 5. Dull, yellow The deposit exhibits no light reflec- EXAMPLE [V tion and gives a matte appearance. 6. Brownish-yellow The deposit has lost its normal A g P g bath was Prepared as follows:

yellow color and gives a matte appearance. 7. Dull, dark brown The deposit is dark brown and gives a matte appearance. I

COMPONENT G/L EXAMPLE I ,P,0 100 A gold plating bath was prepared as follows: Ammonium -P Y- 10 phosphate Au as KAu (CN)2 8 Co as CoSO .7H O 0.l COMPONENT GIL H PO to pH 4.0

Au as N213 Au(SO;,)-,] 8 Na SO, Ammonium amido-poly- 25 3O phosphate NaOH to pH 9.0

Plating on cathodic brass panels was performed at approximately 120F. and 3 ASP with periodic replenishment for a period of 6- /2 hours. Deposits obtained exhibited an average visual rating of 2. Where the same bath was employed without the amido-polyphosphatc, the visual rating of the deposit was 4.

EXAMPLE II A gold plating bath was prepared as follows:

COMPONENT G/L Au as KAu (CN); 8 K HPO 60 Ammonium amidopoly- 20 phosphate G -l 10 pH 7.0

EXAMPLE lll A gold plating bath was prepared as follows:

COMPONENT G/L Au as Na;.[Au(SO 8 Na SO l0 Ammonium amido-poly- 20 phosphate Na P O 30 NaOH to pH 8.5

Plating was performed on cathodic brass panels at approximately lOOF. and 10 ASP. The deposit exhibited a visual rating of 1 compound to a rating of 4 without the cobalt. As the amido-polyphosphate concentration was increased. the concentration of cobalt required to produce a 1 rating gradually increased. At an amido-polyphosphate content of 100 g/l the cobalt concentration required to obtain a 1 rating was in excess of 0.5 g/l, although lesser quantities did improve the brightness to some extent. An analysis of the deposits obtained indicated a cobalt content not exceeding 0.5 percent. Similar results were obtained when nickel as NiSO -6H O was used in place of the cobalt.

EXAMPLE V A bath was prepared in Example 4 but containing 25 g/l amido-polyphosphate and 0.35 g/l cobalt. Plating was performed under the conditions of the prior example and deposits exhibiting a visual rating of l and containing from 0.2 to 0.3 percent cobalt were obtained. The Knoop hardness of these deposits was in the range of 147-159. Without the inclusion of cobalt, hardness values ranged from to 90.

What is claimed is:

1. ln an aqueous gold electroplating bath containing gold in an electrodepositable form adjusted to a pH suitable for plating, the improvement comprising in cluding an amido-polyphosphate compound differing from a normal polyphosphate compound in that one or more of the oxygen linkages between phosphorous atoms is replaced by an -Nl-l-linkage in a sufficient quantity to effect a brighter deposit.

2. The bath of claim 1 wherein said amidopolyphosphatc compound has a P 0 equivalent phosphorus content of from about 65 to weight percent,

and an NH equivalent amide nitrogen content of from about 5 to 12 weight percent.

3. The bath of claim 2 wherein said polyphosphate is an ammonium salt.

4. The bath of claim 3 wherein said amidepolyphosphate compound has an NH equivalent ammonium nitrogen co tent of from about 8 to 22 weight percent.

5. The bath of claim 1 additionally containing at least 1 g/l of normal polyphosphate compound.

6. The bath of claim 1 wherein said amidopolyphosphate compound is present in a concentration of from 1 to 100 g/l.

7. The bath of claim 1 wherein said gold is present in the form of a gold cyanide complex, the pH of the bath is maintained at a value of at least 3.5, and the bath contains at least 0.01 g/l of a metal selected from the group consisting of cobalt and nickel in eo-depositable form.

8. The bath of claim 1 wherein said gold is present in the form of a gold sulfite complex and the pH of the bath is maintained at a value of from 5.5 to 10.5.

9. A method of electroplating a gold layer on a metallic article comprising electrolyzing the solution of claim 1 with said article as cathode.

10. In an aqueous gold electroplating bath containing gold in an elcctrodcpositable form adjusted to a pH suitable for plating, the improvement comprising including an amidopolyphosphatc compound obtained by reacting anhydrous ammonia with phosphorus pentamidooxide and heating the reaction mixture in the presence of excess anhydrous ammonia at a temperature of at least about 150C. for a sufficient period of time to effeet a molecular condensation with loss of ammonia from the product, forming a substantially neutral product in which the atomic ratio of nitrogen to phosphorus is less than 1.5 to l in a quantity sufficient to effect a brighter deposit.

11. The bath of claim 10 wherein said amidopolyphosphate compound has a P 0 equivalent phosphorus content of from about 65 to weight percent, and a NH equivalent amide nitrogen content of from about 5 to 12 weight percent.

12. The bath of claim 11 wherein said amidopolyphosphate is an ammonium salt.

13. The bath of claim 10 additionally containing at least 1 g/l of a normal polyphosphate compound.

14. The bath of claim 10 wherein said amidopolyphosphate compound is present in a concentration of from 1 to g/l.

15. The bath of claim 10 wherein said gold is present in the form of a gold cyanide complex, the pH of the bath is maintained at a value of at least 3.5, and the bath contains at least 0.01 g/l of a metal selected from the group consisting of cobalt and nickel in codepositable form.

16. The bath of claim 10 wherein said gold is present in the form of a gold sulfite complex and the pH of the bath is maintained at a value of from 5.5 to 10.5. 

1. IN AN AQUEOUS GOLD ELECTROPLATING BATH CONTAINING GOLD IN AN ELECTRODEPOSITABLE FORM ADJUSTED TO A PH SUITABLE FOR PLATING THE IMPROVEMENT COMPRISING INCLUDING AN MAMIDO-POLYPHOSPHATE COMPOUND DIFFERING FROM A NORMAL POLYPHOSPHATE COMPOUND IN THAT ONE OR MORE OF THE OXYGEN LINKAGES BETWEEN PHOSPHOROUS ATOMS IS REPLACED BY AN -NH LINKAGE IN A SUFFICIENT QUANTITY TO EFFECT A BIGHTER DEPOSIT.
 2. The bath of claim 1 wherein said amido-polyphosphate compound has a P2O5 equivalent phosphorus content of from about 65 to 80 weight percent, and an NH3 equivalent amide nitrogen content of from about 5 to 12 weight percent.
 3. The bath of claim 2 wherein said amido-polyphosphate is an ammonium salt.
 4. The bath of claim 3 wherein said amido-polyphosphate compound has an NH3 equivalent ammonium nitrogen content of from about 8 to 22 weight percent.
 5. The bath of claim 1 additionally containing at least 1 g/l of normal polyphosphate compound.
 6. The bath of claim 1 wherein said amido-polyphosphate compound is present in a concentration of from 1 to 100 g/l.
 7. THE BATH OF CLAIM 1 WHEREIN SAID GOLD IN PRESENT IN THE FORM OF A GOLD CYANIDE COMPPLEX THE PH OF THE BATH IS MAINTAINED AT A VALUE OF AT LEAST 3.5 AND THE BATH CONTAINS AT LEAST 0.01 G/L OF A METAL SELECTED FROM THE GROUP CONSISTING OF COBALT AND NICKEL IN CO-DEPOSITABLE FORM
 8. The bath of claim 1 wherein said gold is present in the form of a gold sulfite complex and the pH of the bath is maintained at a value of from 5.5 to 10.5.
 9. A method of electroplating a gold layer on a metallic article comprising electrolyzing the solution of claim 1 with said article as cathode.
 10. In an aqueous gold electroplating bath containing gold in an electrodepositable form adjusted to a pH suitable for plating, the improvement comprising including an amidopolyphosphate compound obtained by reacting anhydrous ammonia with phosphorus pentoxide and heating the reaction mixture in the presence of excess anhydrous ammonia at a temperature of at least about 150*C. for a sufficient period of time to effect a molecular condensation with loss of ammonia from the product, forming a substantially neutral product in which the atomic ratio of nitrogen to phosphorus is less than 1.5 to 1 in a quantity sufficient to effect a brighter deposit.
 11. The bath of claim 10 wherein said amido-polyphosphate compound has a P2O5 equivalent phosphorus content of from about 65 to 80 weight percent, and a NH3 equivalent amide nitrogen content of from about 5 to 12 weight percent.
 12. The bath of claim 11 wherein said amido-polyphosphate is an ammonium salt.
 13. The bath of claim 10 additionally containing at least 1 g/l of a normal polyphosphate compound.
 14. The bath of claim 10 wherein said amido-polyphosphate compound is present in a concentration of from 1 to 100 g/l.
 15. The bath of claim 10 wherein said gold is present in the form of a gold cyanide complex, the pH of the bath is maintained at a value of at least 3.5, and the bath contains at least 0.01 g/l of a metal selected from the group consisting of cobalt and nickel in co-depositable form.
 16. The bath of claim 10 wherein said gold is present in the form of a gold sulfite complex and the pH of the bath is maintained at a value of from 5.5 to 10.5. 